I could be wrong, but more likely the deposit is a copper layer that
was probably sputtered (PVD?) on. If you deposit some on steel wire
and put it in a torch flame, you should see a green tinge. (Of
course, you need to decide if the risks you perceive warrant the
benefits of satisfying your curiosity.)

See: http://en.wikipedia.org/wiki/Mirror

Exerpt(s): “….Nowadays, mirrors are often produced by the vacuum
deposition of aluminium (or sometimes silver) directly onto the glass
substrate.… In modern times the mirror substrate is shaped, polished
and cleaned, and is then coated. Glass mirrors are most often coated
with non-toxic silver or aluminium, implemented by a series of
coatings:
tin
silver
chemical activator
copper
paint
The tin is applied because silver will not bond with the glass. The
activator causes the tin/silver to harden. Copper is added for long-
term durability.[13] The paint protects the coating on the back of the
mirror from scratches and other accidental damage….”